As the load on an electric power system grows, the line current increases and energy losses become greater. The load is measured in terms of the product of volts and current, or VA. Therefore, in the past it has been standard practice to increase the voltage level in order to meet growing demands, thereby lowering the current and minimizing the energy losses. This approach may be undesirable, however, because of the potential adverse environmental effects of the higher voltage levels, including high electric fields, radio and television interference, audible noise and induced voltage. If higher voltage levels are not employed to satisfy increased demands, the remaining options available to utilities are: increasing the current of the transmission line, employing load management methods and/or encouraging conservation. Of these options the only one which has no adverse effect on the consumer or the environment is to increase the current carrying capability of the transmission line, even though energy losses may increase slightly.
In order to design and effectively utilize overhead electrical power transmission lines, it is necessary to determine their actual thermal capacity which in turn determines the maximum amount of electrical current that the lines may safely carry. In the past, design ratings for the lines have been derived from theoretical calculations based on pessimistic weather conditions and selected values of conductor temperature. The safe values of conductor temperature are based on line clearance requirements and loss of tensile strength criteria. Weather conditions substantially affect the current carrying capacity of an overhead electrical power line. Theoretical calculations are normally based on assumptions of low wind speeds perpendicular to the conductor, high ambient temperatures and maximum solar radiation, consequently the calculation for arriving at the design rating is based on the assumption that the weather will have a minimum cooling effect on the conductor while maximizing the amount of heat absorbed by the conductor. This ensures that the line temperature will be the highest attainable when the line is carrying the rated load in order to prevent the sag of the line from exceeding a preselected safe clearance above the ground, or in order to prevent the conductor from losing more than the acceptable loss of tensile strength.
It has been found that the conservative theoretical approach described above sometimes results in line temperatures greater than the calculated value. Numerous reasons exist for this disparity; for example, in the event the wind speed is lower than the assumed value, or if the wind is blowing parallel to the conductor, and both the ambient temperature and solar radiation are greater than the assumed values, then the line temperature, and consequently the line sag will be greater than expected.
In order to obtain more accurate design ratings, some utility companies in the past have established weather stations at various locations in the general vicinity of the transmission lines in order to monitor the weather and thus obtain more reliable climatological data which is used to improve the calculations for arriving at the design ratings. This approach to the problem is less than completely satisfactory for two reasons. First, the weather information recorded at a single location is not necessarily representative of the weather along an entire transmission line. Secondly, since weather is variable in both time and location, it is impossible to accurately calculate how the conductor temperature will respond to these variable conditions.
It would therefore be desirable to measure the actual temperature of the transmission line on a real-time basis, since this would allow rating the line as function of the prevailing weather conditions rather than based on assumed pessimistic weather conditions or weather forecasts. Measuring the actual conductor temperature of the line provides two advantages. First, line capacities greater than the design rating are presently available for approximately 90% of the time during the year. Secondly, utilities can now predict when a transmission line cannot safely carry the design rated load. A system for directly monitoring the line temperature and weather parameters and for determining the maximum capacity of each transmission line would afford an immediate, low cost, minimum risk solution to a capacity deficiency problem which may be particularly acute in areas where there is uncertainty in the load growth rates, public resistance to acquiring rights-of-way for overhead lines and inadequate capital funding.
U.S. Pat. Nos. 4,268,818 and 4,420,752 disclose devices for monitoring, on a real-time basis, the conductor temperature, ambient temperature and line current of an overhead power line. The devices shown in these patents required that the conductor temperature sensor be installed at some distance away from the device so as not affect the measured conductor temperature. These devices were adaptable only to a relatively small range of conductor sizes, without changing the clamp used to attached the device to the conductor line. Since hundreds of different sizes are currently in service, these devices have limited application in those cases where the transmission line designer desires to move the device from location to location and install them on different conductor sizes.
As discussed in these prior patents, the load carrying capability of a power line may be restricted by its thermal rating; the thermal rating is the maximum current the line is capable of carrying and is normally based on the maximum allowable or safe conductor temperature and assumed, worst-case weather conditions. The starting point in establishing this rating is to select a safe value of conductor temperature such that the line clearance requirements and loss of conductor tensile strength criteria are not exceeded.
In establishing the thermal ring, it is necessary to select a safe value of conductor temperature, which is normally based on clearance requirements, i.e., the distance from the power line to the nearest point on the earth or to an object under the power line. When a steady-state current is applied to a transmission line conductor during steady-state weather conditions, the line heats up due to the internal generated within the conductor or the I.sup.2 R losses. This causes the conductor temperature to increase above the ambient air temperature and the line begins to sag from its original unheated position to a lower position, since the length of the line changes. Assuming the line is fixed at each end to a tower or similar structure, the tension will also decrease, since the length of the line becomes longer when heated. The final value of the sag that a conductor reaches for a set of weather conditions and current is important since clearance requirements must not be exceeded in order to protect against objects on the ground coming in contact with high voltage lines. Additional factors affecting the final sag of the line include mechanical creep and elevated temperature creep.
The mechanical creep is a function of time and tension, whereas elevated temperature creep is a function of line tension, conductor temperature and time. Both of these factors also increase the sag of a line. When the initial stringing tension is known, the mechanical creep can be determined as a function of time only. Elevated temperature creep is a function of line tension, conductor temperature and time. Based on the above, if the conductor temperature is monitored, then the actual line sag or clearance can be determined on real-time basis.
Another factor affecting the choice of the maximum conductor temperature is the loss of conductor tensile strength as the conductor is heated. For any specific conductor size, type (i.e., ACSR, aluminum, copper, etc.) and stranding, the loss of strength is dependent on conductor temperature and time. The loss of tensile strength is accumulative, that is, the more the conductor is heated, the greater the loss of strength and the initial strength is never regained.
As is apparent from the foregoing, all of the factors which limit the current rating to a safe value are a function of conductor temperature. Thus, if the line conductor temperature and weather conditions are monitored, then the maximum real-time current will be substantially greater than the conservative design rating for a large portion of time during the year.
A primary object of the present invention is to overcome each of the deficiencies of the prior approach to establishing the thermal rating of power lines. This, and further objects of the invention will become clear or will be made apparent during the course of the following description of a preferred embodiment of the invention.